1. Field of the Invention
This invention pertains to a heater pad with a PTC (positive temperature coefficient) element and a buss system to equalize the current path distances.
2. Description of the Prior Art
In the prior art, PTC (positive temperature coefficient) heaters, such as those disclosed in U.S. Pat. Nos. 4,857,711 and 4,931,627 to Watts, have a resistance which increases in response to increasing temperatures. This fundamentally reduces thermal energy output in view of a substantially constant voltage applied across this resistance, thereby tending to prevent overheating, and is therefore useful in applications with varying ambient temperatures, such as automotive mirror defrosting. Users in several applications desire a heater with both terminals across a single face of the heater in order to simplify electrical connections and to accommodate standard electrical circuitry. However, such a configuration often results in uneven resistance through the various electrical paths thereby resulting in uneven heating across the heating surface, increased current draw, and increased buss width requirements.